neural spine bifurcation in sauropod dinosaurs of the - PalArch

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Wedel & Taylor, Neural Spine Bifurcation in Sauropods PalArch’s Journal of Vertebrate Palaeontology, 10(1) (2013) Figure 19. The sixth cervical vertebrae of Diplodocus carnegii CM 84/94, Suuwassea emilieae ANS 21122, and Apatosaurus sp. CM 555 in left lateral view, scaled to the same centrum length. Actual centrum lengths are 442 mm, 258 mm, and 327 mm, respectively. Diplodocus carnegii modified from Hatcher (1901: plate 3), reversed left to right for ease of comparison. Suuwassea emilieae from a photo provided by Jerry Harris; the same photo also appears as Harris (2006c: text-figure 7B). Apatosaurus photographs by Mathew Wedel, digitally composited by Michael Taylor. tosaurus cervicals also have tall, narrow neural spine tips that somewhat resemble those of Suuwassea. Summary – The cervical vertebrae of Suuwassea differ from those of Diplodocus in almost every detail, and there is no evidence from presacral morphology that Suuwassea is a juvenile of Diplodocus. Apatosaurus – According to Gilmore (1936: 196), C6 of A. louisae CM 3018 has a centrum length of 440 mm and a cotyle diameter of 150 mm, and therefore an EI of 2.9. C6 of A. parvus UWGM 15556 was not preserved, but C5 and C7 have EIs of 2.6 and 2.4, respectively (Gilmore, 1936: 196). The vertebrae between the fifth and seventh positions in Apatosaurus are therefore consistently shorter than C6 in Suuwassea (EI of 3.4), but more similar in proportions than the equivalent vertebrae in Diplodocus. As noted above, some cervical vertebrae of Apatosaurus have prezygapophyseal rami shaped like anterodorsally-projecting pedestals, and forward-leaning, chimney-shaped neural spine tips (Gilmore, 1936: plates 24 and 31), but in no known vertebrae of Apatosaurus do these characters reach the same degree of expression as in Suuwassea. Note that Lovelace et al. (2008) recovered Suuwassea as an apatosaurine, but not as Apatosaurus. The most striking difference between Suuwassea and Apatosaurus is that Suuwassea lacks the immense, low-hanging cervical ribs that are diagnostic for Apatosaurus (see Upchurch et al., 2005: 80-81). The cervical ribs of Suuwassea are short, as in other diplodocoids, and do not extend past the end of the centrum of the vertebra on which they originate, but they are neither enlarged nor set well below the centrum as in Apatosaurus. This cannot be explained as an result of ontogeny because vertebrae of subadult Apatosaurus with unfused neural arches and cervical ribs nevertheless have greatly enlarged parapophyses to support the latter (figure 18). Summary – The cervical vertebrae of Suuwassea are more similar to those of Apatosaurus than those of Diplodocus, but they differ in several important characters that cannot be interpreted as ontogenetically labile. On gross morphology alone, it is very unlikely that Suuwassea represents a juvenile of either taxon. The case for synonymy grows even worse when skeletochronology is considered, as discussed in the next section. Ontogenetic status of Suuwassea – The Suuwassea holotype ANS 21122 can be assessed for four of the non-histological criteria of skeletal maturity discussed above: 1) Sheer size; 2) Fusion of the neural arches and centra; 3) Fusion of the cervical ribs to their respective vertebrae; 4) Fusion of the scapula and coracoid. We will ignore sheer size for reasons explained above, and discuss the other evidence in turn. The neural arches are fused in the cervicals and dorsals but unfused in most of the caudals. Harris (2006c: 1107): “Of all the caudal vertebrae preserved in ANS 21122, only the distal, ‘whiplash’ caudals are complete. All the remaining vertebrae consist only of vertebral bodies © PalArch Foundation 22
Wedel & Taylor, Neural Spine Bifurcation in Sauropods PalArch’s Journal of Vertebrate Palaeontology, 10(1) (2013) [i.e. centra] that lack all phylogenetically informative portions of their respective arches. On the proximal and middle caudals, this absence is due to lack of fusion as evidenced by the deeply fluted articular surfaces for the arches on the bodies. In contrast, the arches on the most distal vertebrae that retain them are seamlessly fused, but everything dorsal to the bases of the corporozygapophyseal [i.e. centrozygapophyseal] laminae are broken.” It is interesting that the unfused arches in the proximal and middle caudals are bracketed by fused arches both anteriorly (in the dorsals) and posteriorly (in the distal caudals). This shows that neural arch fusion in Suuwassea was not a simple “zipper” that ran from back to front, as in crocodilians (Brochu, 1996) and phytosaurs (Irmis, 2007), or front to back. The sequence of neural arch fusion cannot be determined based on the one available skeleton of Suuwassea, but clearly the anterior and middle caudals would have fused last, at least in this individual. The cervical neural arches are all fused, but some of the cervical ribs are partly fused or unfused (Harris, 2006c). In C3, the left cervical rib is not attached, and the right one is attached at the parapophysis but not fused. In C5, the ribs are attached, not fused at the parapophyses, and fused at the diapophyses (this may be the first time that anyone has documented which of the two attachment points fused first within a single cervical rib in a sauropod). In C6, the ribs are fused at both attachment points. C7 lacks the ribs, but their absence appears to be caused by breakage rather than lack of fusion. One fragmentary posterior cervical of uncertain position is missing the diapophyses but has one rib fused at the parapophysis. Finally, the scapula-coracoid joint is unfused (Harris, 2007), but that is often the case even for substantially ‘adult’ sauropods such as Giraffatitan brancai MB.R.2181 and Apatosaurus excelsus YPM 1980. Based on the lack of fusion in the caudal neural arches, anterior cervical ribs, and scapulocoracoid joint, Suuwassea holotype ANS 21122 was not fully mature. However – and this is absolutely crucial for the synonymization hypothesis – the Suuwassea specimen already has a greater degree of cervical element fusion than Diplodocus carnegii holotype CM 84/94 (which has unfused ribs back to C5) and Apatosaurus CM 555 (which has unfused arches back to C8 and unfused ribs throughout), both of which have attained essentially ‘adult’ morphology. So if Woodruff & Fowler (2012) are correct in identifying Suuwassea as a juvenile of a known diplodocid, the ontogenetic clock has to run forward from CM 555 and CM 84/94, through a Suuwassea-like stage, and then back to normal Apatosaurus or Diplodocus morphology. This is sufficiently unlikely to not warrant further consideration. The unfused arches in the Suuwassea caudals are especially interesting because most of the cervical ribs are fused. This is in contrast to D. carnegii CM 84/94, in which all the neural arches are fused but the anterior cervical ribs are not. So the developmental timing in Suuwassea is dramatically different than in D. carnegii, which is a further problem for the synonymization hypothesis: Suuwassea doesn’t belong in the same ontogenetic series as Diplodocus, contra Woodruff & Fowler (2012: figures 3 and 9) – if the timing of the various fusions differs between the taxa, there is no basis for assuming that the hypothetical ontogenetic bifurcation would follow the same rules. In summary, the entire rationale for the taxonomic arguments of Woodruff & Fowler (2012) – that Suuwassea has incompletely bifurcated neural spines because it is a juvenile – turns out be an illusion caused by not taking serial variation into account. Suuwassea ANS 21122 probably is a subadult, based on the unfused caudal neural arches, but its cervical vertebrae already show the expected adult morphology in neural arch fusion, cervical rib fusion (except the most anterior), and – most importantly – neural spine bifurcation. The taxonomic distinctness of Suuwassea and the nearly adult stage of the holotype are further supported by the histological work of Hedrick et al. (In Press). Is Haplocanthosaurus a Juvenile of a Known Diplodocid? Although Woodruff & Fowler (2012) argue at length that Suuwassea is a juvenile of another taxon, they also suggest that the same might be true of other Morrison Formation sauropods. From their Conclusions section (Ibidem: 9): ”Just as particularly large diplodocid specimens (e.g. Seismosaurus; Gillette, 1991) have been more recently recognized as large and potentially older individuals of already recognized taxa (Diplodocus; Lucas et al., 2006; Lovelace et al., © PalArch Foundation 23
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